The word virus means poison, and before the relation of bacteria, fungi, spirochetes and protozoa to disease was recognized it was customary to use it in a noncommittal manner to designate the causative agents responsible for certain maladies. As the etiologic agent of one disease after another was discovered, the word virus was still used at times to indicate a known infectious agent; for instance, the expression virus of syphilis is equivalent to Spirochaeta pallida. In addition to this new usage, the word was still employed to designate unknown or undiscovered etiologic agents. In view of Koch's postulates, infectious agents that had not been seen and cultivated on lifeless laboratory mediums were considered to be undiscovered or unknown, in spite of the fact that investigators were able to experiment with some of them, e. g., the viruses of cowpox and smallpox, and to use them for prophylactic purposes. Thus at